It is known that ultraviolet A (UVA) in a wavelength region of 320 to 400 nm and ultraviolet B (UVB) in a wavelength region of 290 to 320 nm reach down to the ground without being sufficiently absorbed and scattered by the ozone layer, and cause various negative effects on skin. For example, it is known that UVB induces inflammation and accelerates skin aging and that WA accelerates melanin production and causes blotches, freckles, and so on.
Cosmetics for preventing such negative effects on skin have been developed and examined in various ways. For example, there has been made an attempt to prevent skin from excessively getting exposed to ultraviolet rays through the use of a compound having an aromatic ring such as a benzene ring, i.e., the so-called UV absorbing agent (see Patent Document 1 and 2, for example).
However, as many of the UV absorbing agents used in these documents are oils, the agents have low solubility in water, and thus their forms as cosmetics are sometimes limited. When these UV absorbing agents are used as cosmetics, a refreshing tactile sensation is hardly obtained. Because of this, there has been a limit to use these UV absorbing agents as ultraviolet protection cosmetics frequently used in summer.
Meanwhile, there has been made a report on an attempt to prevent skin from excessively getting exposed to ultraviolet rays by using inorganic fine particles such as zinc oxide fine particles or titanium oxide fine particles (see Patent Documents 3 and 4, for example). It is known that zinc oxide, titanium oxide, and the like have great UV protective effects. In particular, it is known that titanium oxide has a high level of UVB shielding ability and zinc oxide has a high level of UVA shielding ability.
However, making titanium oxide fine particles or zinc oxide fine particles into an uniform dispersion in an aqueous solution tends to be hindered because of their surface activities. Accordingly, adding titanium oxide or zinc oxide to a cosmetic base leads to nonuniform dispersion. As a result, the cosmetic that has become a cloudy solution is applied to a face or a body, which causes problems including the face or the body turns whitish and the UV protective effect of the cosmetic is diminished. Further, there has been a problem in that titanium oxide or zinc oxide affects other compounding ingredients, such as a thickening agent, in a cosmetic and the function of the cosmetic deteriorates, for example.
In order to disperse uniformly titanium oxide or zinc oxide in an aqueous solution, a method of making titanium oxide or zinc oxide into a superfine particles and a method of adding a surface active agent to the aqueous solution have been worked out. An example of the method of adding of a surface active agent includes a method of preparing a neutral titania sol by compounding a water-soluble high-molecular compound (polyvinyl alcohol or the like) as a dispersion stabilizing agent into an aqueous acid solution containing titania fine particles and then compounding an alkaline solution as a neutralizing agent into the resultant solution (Patent Document 5); however, when a cosmetic additive agent was added, maintaining the stability of the neutral titania sol was difficult.
Moreover, to uniformly disperse titanium oxide or zinc oxide in a solution, there has also been made a report of treating the surfaces of the inorganic fine particles with a silicone-based copolymer, a particular phosphate ester, or the like (see Patent Documents 6 and 7, for example); however, the treatment is insufficient in effect.
The inventors of the present invention have produced metal oxide fine-particle composites by dissolving a particular short-chain fatty acid or polymer and so on and a metal salt in a water-soluble organic solvent or a mixed solution of water and a water-soluble organic solvent and then neutralizing the metal salt or reducing the metals in the metal salt (Patent Document 8). Among such metal oxide fine particle composites, zinc oxide fine particles can be dispersed uniformly in the aqueous solution and have both transparency and ultraviolet absorbency. On the other hand, in the case of the titanium oxide fine-particle composite, the dispersibility in an aqueous solution is not enough so that both transparency and ultraviolet absorbency could have not been sufficiently attained. Further, there are also problems that the titanium oxide fine particles cannot be redispersed when once the particles aggregate because of the instability in the composite state and the dispersion of the composite has high viscosity.
In view of such circumstances, it is desired that a titania fine-particle composite, which is excellent in transparency and ultraviolet-absorbing effect and is stable in a composite state, be developed because the composite has broad utility.
Citation List
Patent Document
    [Patent Document 1] JP 2001-207060 A    [Patent Document 2] JP 2006-160651 A    [Patent Document 3] JP 2004-203768 A    [Patent Document 4] JP 2003-096437 A    [Patent Document 5] JP 63-123815 A    [Patent Document 6] JP 2007-291094 A    [Patent Document 7] JP 2005-232069 A    [Patent Document 8] WO 2007/057997 A1